1. Field of the Invention
The present invention relates to a magnet roller, a magnetic particle-support member, a development device, a process cartridge, and an image forming apparatus used for copying machines, facsimiles, printers or the like. More specifically, the present invention relates to a magnetic particle-support member, by which an electrostatic latent image on an image carrier is developed with a developer including toner and carrier having magnetic particles to form a toner image, a magnet roller using such a magnetic particle-support member, and a development device having such a magnetic particle-support member. In addition, the present invention relates to a process cartridge and an image forming apparatus having such a development device.
2. Description of Related Art
In general, a development device has a developer-support member as a magnet particle-support member, which is configured to convey developer to a development area facing an image-support member and develop an electrostatic latent image formed on the image-support member to form a toner image. The developer-support member includes, for example, a cylindrical non-magnetic sleeve, that is, a development sleeve as well as a magnetic field-generation device, for example, a magnet roller, which generates a magnetic field to form raised portions or ears of the developer on a surface of the development sleeve.
The developer has toner and carrier including magnetic particles. When forming the raised portion of the developer, the carrier of the developer is raised on the development sleeve along magnetic field lines generated by the magnet roller and the charged toner is attached to the raised carrier.
The magnet roller has a side surface in which at least one magnet is disposed or buried and the magnet forms a plurality of magnetic poles. The magnet forming each of the magnetic poles is formed in a rod-like shape and, in particular, forms a development pole to form the raised portion of the developer on a part corresponding to a development area of the development sleeve. By rotating at least one of the development sleeve and the magnet roller, the developer raised by the pole is moved in a circumferential direction of the development sleeve or the magnet roller. In general, the development sleeve has a surface which is processed by a roughening process such as a groove processing, a sandblast processing or the like so that the developer is easily conveyed. Such a groove processing and a sandblast processing are performed in order to prevent reduction of image density occurring when the developer slips and is interrupted on the surface of the development sleeve which rotates at high speed.
FIG. 19 shows a conventional development device. Reference number 200 denotes a development device. The development device 200 has a developer-support member 204 configured to convey the developer to the development area facing an image-support member 211 and develop an electrostatic latent image formed on a surface of the image-support member 211 to form the toner image. In addition, the developer-support member 204 includes a cylindrical development sleeve 202 and a magnet roller 201 which is contained in the development sleeve 202 and generates a magnetic field so as to form raised portions of a developer 208 on a surface of the development sleeve 202. On the developer-support member 204, when the developer 208 is raised, magnetic carrier of the developer 208 is raised on the development sleeve 202 along magnetic field lines generated by the magnet roller 201 and toner of the developer 208 is attached to the raised magnetic carrier.
Such a development device 200 includes a developer container 207 to contain the above-mentioned developer, an agitating screw 206 which agitates the developer contained in the developer container 207, and a developer control member 205 which uniformly controls an amount of the developer picked up on the developer-support member 204.
The development device 200 shown in FIG. 19 is provided with the developer container having a pair of developer-containing tanks 207a, 207b and the agitating screw 206 having a pair of agitating screw members 206a, 206b. The developer in the development device 200 moves in an axial direction of the agitating screw 206 in the developer container 207. The toner supplied from an end portion of the developer-containing tank 207b which is away from the developer-support member 204, is conveyed to another end portion of the developer-containing tank 207b along an axial direction of the agitating screw member 206b by the agitating screw member 206b while being agitated with the developer. The developer is moved from the other end portion of the developer-containing tank 207b to the other developer-containing tank 207a which is close to the developer-support member 204. The developer moved to the developer-containing tank 207a is picked up to the surface of the development sleeve 202 by a magnetic force of the magnet roller 201. That is to say, the developer is attached to the surface of the development sleeve 202. After that, an amount of the developer is uniformly controlled by the developer control member 205, and then the developer is conveyed to a development area where the image-support member 211 and the developer-support member 204 are disposed to face each other with an interval. An electrostatic latent image formed on the image-support member 211 is developed with the developer 208 to form a toner image.
In recent years, colorization of electronic copying machines and printers has been advanced. Since a color copying machine requires four development devices, a reduced size of the development devices is desired to reduce the size of machines. In order to reduce the size of the development devices, the size of developer-support members contained each development device is also required to be reduced. The size reduction of the development device, however, causes problems as follows.
(1) In the development device, in order to prevent the developer from attaching to the image-support member as an electrostatic latent image-support member, it is necessary for a development main pole and adjacent poles to have high magnetic forces (generally, 100 mT or more on the developer-support member). However, because a volume of each magnet of the small size developer-support member is reduced, it is difficult for the small size developer-support member to generate a high magnetic force.
(2) In the case of the small size developer-support member, the developer-support member has low work-stiffness so that the developer-support member is easily deformed when being processed by a sandblast processing, or the like which is used as a conventional surface processing. Accordingly, it is difficult to achieve high accuracy of the developer-support member.
(3) In a case of the developer-support member having a small diameter, the magnetic force greatly changes due to a distance from the support member so that it is difficult to suck and stably maintain the developer onto the developer-support member.
To solve the above problems, for example, Japanese Patent Application Publication No. H05-033802 discloses a method in which a magnetic field is virtually oriented in a multi-pole state so that multi poles are disposed to form magnetic poles in spite of an integral construction. However, there are problems in that the main pole may achieve only a magnetic force of about 90 mT on the developer support member, in that complex structures are required in a die because of the multi-pole configuration, and the like.
Japanese Patent Application Publication No. 2000-068120 proposes configurations where a magnet block which is made of a plastic resin and metal powder having high magnetism such as metal powder made of rare earth alloys is attached to a part of a roll made of a plastic magnet.
However, a large quantity of magnetic materials are blended in order to obtain high magnetism, so that such a magnet block has low mechanical strength. Therefore, even after the magnet roll which is formed by the magnet block together with a roll made of a plastic magnet is obtained, the magnet roll is easily affected by, for example, damage or cracks which occur when being treated. In addition, after the magnet roll is attached to the image forming apparatus, the magnet block is possibly affected by shocks causing damage or cracks so that image failure or lack of durability occurs. Furthermore, these problems specifically affect application to long roll requirements, for example, in A3 paper-enabled machines.